Direction finding cell phones

ABSTRACT

A direction finding system comprising: at least one first hand holdable unit comprising circuitry that transmits a radio beacon signal; and at least one second hand holdable unit having a display screen and comprising direction finding (DF) circuitry that receives a radio beacon (RB) signal transmitted by a first unit of the at least one first unit and determines from the received radio beacon signal an azimuth angle for the location of the first unit; wherein the controller generates a display on the display screen responsive to the azimuth angle that indicates a location of the first unit.

RELATED APPLICATION

This application claims the benefit under 119(e) of U.S. Provisionalapplication 60/383,594, filed May 29, 2002, the disclosure of which isincorporated by reference.

FIELD OF THE INVENTION

The invention relates to cell telephones and in particular to celltelephones adapted so that a cell telephone user can establish contactwith another cell phone user responsive to the latter's position.

BACKGROUND OF THE INVENTION

Circumstances arise in which a person is interested in establishingcontact with another person he or she sees but does not know. Often, insuch circumstances it is inconvenient, improper or perhaps too dauntingfor the person to walk up to the other person, introduce himself orherself and make the other's acquaintance. For example, a person mightsee another person in a crowd to whom he or she is attracted but cannotapproach because the other person is involved in an activity that itwould be rude to interrupt. Or a person might want to make theacquaintance of another person who is presenting a topic to a group ofpeople but cannot wait until the other person finishes in order tointroduce himself or herself to the other person. In such circumstancesit would be advantageous for the person to be able to relatively easilyestablish discrete contact with the other person and exchange sufficientinformation to enable the two people to get in touch with each other ata later opportunity if the exchanged information warrants.

Benefon of Finland markets a GSM phone that comprises a GPS receiver,which provides coordinates of the location of a user of the phone. TheGSM phone is described in the company's product data sheets [online];[retrieved on May 21, 2003]; retrieved from the internet<URL:www.benefon.com/products/esc/product_data.htm>. Users of the phonesmay determine locations of friends who also use the phones by requestingand receiving via SMS messages transmitted over the GSM network, the GPScoordinates of their respective locations. Coordinates of friends that auser of a Benefon phone receives are displayed on the phone's displayscreen over a background of a suitable map to indicate the friends'locations. However, the phone cannot generally be used by its user toapproach and/or determine the location of a stranger, since access toanother person's GPS coordinates requires that the other person's phonenumber be known, and presumably a stranger's phone number is not known.

SUMMARY OF THE INVENTION

An aspect of some embodiments of the present invention relates toproviding a communication system that enables a first person todiscretely exchange information with a second person responsivesubstantially only to the second person's spatial location in the firstperson's field of view. Optionally, the first person does not have toknow the second person or any of the second person's personal data inorder to establish contact and exchange information with the secondperson.

According to an aspect of some embodiments of the present invention thecommunication system comprises cell phones, hereinafter referred to as“radar phones”, at least some of which comprise circuitry fortransmitting a radio beacon (RB) signal and at least some of whichcomprise direction-finding (DF) circuitry. The RB and DF circuitry inthe cell phones are in addition to conventional circuitry for supportingconventional cell phone telephony.

In accordance with an embodiment of the invention, a radar phoneoperated in a DF mode by a person transmits at least one optionallynon-directional signal, hereinafter an “interrogation signal”. Theinterrogation signal has sufficient intensity so that it is readilyreceivable by radar phones operating in a radio beacon (RB) mode thatare carried by people located in at least a portion of the operator'sfield of view in a neighborhood of the operator. Each RB mode phone thatreceives the interrogation signal responds by transmitting a signal,i.e. a “radio beacon signal”, comprising a CW signal and ID data thatcan be used to identify the radar phone transmitting the RB signaland/or its operator. The DF and RB phones transmit and receiveinterrogation and radio beacon signals at a frequency or frequencies ina suitable frequency band, hereinafter a “DF channel”, which may forexample be a portion of the bandwidth used for conventional cellulartelephony or an ISM band.

RB signals transmitted by the RB phones that are received by the DFphone are processed by the DF phone to determine azimuths and optionallyranges for locations of the transmitting RB phones. Once the DF phonedetermines azimuths and optionally ranges for the RB phones, positionsof the RB phones are indicated on a display screen of the DF phone.Optionally, the positions are indicated by suitable icons, hereinafter“RB icons”, displayed on the DF phone screen against a backgrounddisplay of a radar screen. The location of the RB icons on the “radarscreen” correspond to the spatial locations of the RB phones relative tothe DF phone orientation.

By scanning his or her field of view and the radar screen display of RBicons, the operator can associate a given RB icon with a correspondingperson in the field of view. Using any of various methods known in theart, the operator selects an RB icon on the screen corresponding to aparticular person in the operator's field of view with whom the operatorwould like to establish contact.

Following selection of the icon, the operator may then use the DF phoneto transmit, optionally over the DF channel a suitable SMS message tothe selected person's radar phone providing data that would enable theselected person, if the selected person wishes, to contact the operator.The SMS message comprises ID data comprised in the RB signals receivedfrom the selected person's radar phone that identifies the selectedradar phone. Radar phones that receive the SMS message operateresponsive to the ID data so that only the radar phone for which the SMSmessage is intended accepts the message and stores it in a cell phonememory. In some embodiments of the invention, if the ID data transmittedby the selected person's RB phone comprises a cell phone number, theoperator may control his or her DF phone to transmit an SMS message tothe selected RB phone by conventional cell telephony.

In accordance with an aspect of some embodiments of the presentinvention, the direction finding circuitry in a radar phone comprises aWatson-Watt direction finder having two identical antennae. Thedirection finding circuitry processes signals sensed by both antennae todetermine azimuths and ranges using methods known in the art. Celltelephony circuitry uses one of the antennae to transmit and receiveconventional telephony signals. The radio beacon transmitting circuitrymay be any suitable transceiver known in the art that can receivesignals transmitted over the DF channel and transmit a suitable radiobeacon for a radar phone operating in an RB mode, in accordance with anembodiment of the present invention.

The DF and/or RB circuitry in a radar phone is coupled to the phone'santennae via switching circuitry that protects the DF and/or RBcircuitry from being damaged by conventional telephony signals generatedby the cell phone. In accordance with some embodiments of the presentinvention the switching circuitry automatically prevents operation ofthe phone in the DF or RB mode from interfering with conventional celltelephony.

There is therefore provided in accordance with an embodiment of thepresent invention a direction finding system comprising: at least onefirst hand holdable unit comprising circuitry that transmits a radiobeacon signal; and at least one second hand holdable unit having adisplay screen and comprising direction finding (DF) circuitry thatreceives a radio beacon (RB) signal transmitted by a first unit of theat least one first unit and determines from the received radio beaconsignal an azimuth angle for the location of the first unit; wherein thecontroller generates a display on the display screen responsive to theazimuth angle that indicates a location of the first unit. Optionally,the direction finding circuitry comprises Watson Watts direction findingcircuitry.

Optionally, the at least one second unit comprises for receiving RBsignals, a first antennae and a second antenna electrically connected tothe Watson Watts direction finding circuitry. Optionally, a differencein signal attenuation between the electrical connections of the antennaeto the Watson Watts circuitry is less than about 0.3 dB. Optionally, adifference in signal attenuation between the electrical connections ofthe antennae to the Watson Watts circuitry is less than about 0.2 dB.Optionally, a difference in signal attenuation between the electricalconnections of the antennae to the Watson Watts circuitry is less thanabout 0.1 dB.

In some embodiments of the present invention, the antennae have anelectrical length less than about one fifth the wavelength of a carrierwave of the radio beacon signal. Optionally, the antennae have anelectrical length equal to about one sixth the wavelength of the carrierwave of the radio beacon signal.

In some embodiments of the present invention, the two antennae arespaced apart by a distance less than about one fifth of the carrierwavelength. Optionally, the two antennae are spaced apart by a distanceequal to about one eighth of the carrier wavelength.

In some embodiments of the present invention, the Watson Watts circuitrydetermines the azimuth from a difference between amplitude and/or phaseof the received RB signal at the antennae.

In some embodiments of the present invention, the at least one firstunit comprises circuitry and apparatus that provides conventional cellphone telephony. Optionally, the at least one first unit comprises acommon antenna for transmitting RB signals and for cell phone telephonyfunctions. Optionally, the at least one first unit comprises a switchcontrollable to selectably, electrically connect the common antenna tothe radio beacon circuitry or the cell phone circuitry.

In some embodiments of the present invention, the at least one secondunit comprises circuitry and apparatus that provides conventional cellphone telephony. Optionally, the at least one second unit comprises aswitch controllable to selectably, electrically connect the firstantenna to the direction finding circuitry or the cell phone circuitry.

In some embodiments of the present invention, the RB signals comprise acarrier wave and the at least one first unit and the at least one secondunit comprise a filter that blocks electromagnetic energy at a frequencyof the carrier wave from reaching the cell phone circuitry.

In some embodiments of the present invention, the direction findingcircuitry determines a range for the first unit of the at least one unitresponsive to the received RB signal. Optionally, the direction findingcircuitry determines a DC level of the RB signal. Optionally, thecontroller determines the range responsive to magnitude of the DC level.The controller optionally, generates the display responsive to thedetermined range.

In some embodiments of the present invention, the at least one secondunit comprises circuitry for transmitting signals to the at least onefirst unit and the at least one first unit comprises circuitry forreceiving signals transmitted by the at least one second unit.

Optionally, a second unit of the at least one second unit transmits aninterrogation signal responsive to which a first unit of the at leastone first unit that receives the interrogation signal transmits an RBsignal. Optionally, subsequent to transmitting the interrogation signalthe second unit transmits at least one additional interrogation signal.Optionally, the at least one additional interrogation signal istransmitted following a delay period that begins after a last RB signalreceived by the second unit that is transmitted by the at least onefirst unit responsive to the preceding interrogation signal. In someembodiments of the present invention, each interrogation signaltransmitted by the second unit comprises ID data specific to a user ofthe second unit.

Optionally, each of the at least one first unit is programmable withpreference data specific to a user of the first unit and if it receivesan interrogation signal transmitted by the second unit it transmits anRB signal responsive thereto only if the ID data in the transmittedinterrogation signal matches preference data with which it isprogrammed.

In some embodiments of the present invention, the transmitting circuitryof each first unit transmits its RB signal following a predetermineddelay period after receipt of an interrogation signal. Optionally, thepredetermined delay period for each first unit is chosen from pluralityof different delay periods so as to reduce a probability that any two ofthe first units that receive a same interrogation signal have a samedelay period. Additionally or alternatively, the transmitting circuitryof the first unit dithers its predetermined delay period.

In some embodiments of the present invention, each first unit isprogrammable so that RB signals transmitted by the first unit comprisesID data specific to a user of the first unit. Optionally, each unit ofthe at least one second unit is controllable by its user to transmit asignal comprising ID data that it receives in an RB signal from a givenfirst unit whose location is indicated in the display, which given firstunit is selectable by the user from the display. Optionally, the secondunit is programmable with preference data specific to the second unit'suser and wherein the location of a first unit is indicated on the screenonly if ID data in the RB signal received from the first unit matchespreference data with which it is programmed.

In some embodiments of the present invention, the display indicating aposition of a first unit comprises an icon representing the first unitdisplayed against a background of a radar screen and wherein a locationof the icon on the radar screen corresponds to a location of the firstunit relative to the orientation of the second unit. Optionally, a firstunit of the at least one first unit is programmable so that RB signalsthat it transmits comprises data encoding at least one visual cuecharacteristic of the user of the first unit. Optionally, the controllerof the at least one second unit displays on the screen, in associationwith an icon representing a first unit, a visual cue of the at least onevisual cue encoded in an RB signal it receives from the first unit.

In some embodiments of the present invention, the RB signals comprise acarrier wave having a frequency in a range from about 800 MHz to about900 MHz.

In some embodiments of the present invention, a second unit of the atleast one second unit has an effective maximum range less than or equalto about 200 meters for receiving RB signals transmitted by a first unitthat are useable to determine an azimuth for the first unit. Optionally,the maximum range is less than or equal to about 100 meters. Optionally,the maximum range is less than or equal to about 50 meters.

There is further provided a communication system comprising: a pluralityof cellular phones each of which comprises a display screen a GPSreceiver that determines spatial coordinates for the phone's positionand a transceiver for transmitting non-telephony signals; wherein thetransceiver of a first phone of the plurality of phones is controllableto transmit an interrogation signal responsive to which the transceiverof a second phone of the plurality of phones that receives theinterrogation signal transmits a signal comprising GPS coordinates ofthe second phone; and if the first phone receives the signal transmittedby the second phone, it displays a position icon responsive to the GPScoordinates on the first phone's screen that indicates a location of thesecond phone.

Optionally, each phone comprises a compass that generates signalsresponsive to a heading of an operator of the phone and wherein thesecond phone displays responsive to the compass signals, and togetherwith the position icon, a heading icon indicating the heading of thesecond phone's operator. Optionally, the compass comprises a GPScompass. Additionally or alternatively, the compass comprises a magneticcompass.

BRIEF DESCRIPTION OF FIGURES

Non-limiting examples of embodiments of the present invention aredescribed below with reference to figures attached hereto, which arelisted following this paragraph. In the figures, identical structures,elements or parts that appear in more than one figure are generallylabeled with a same numeral in all the figures in which they appear.Dimensions of components and features shown in the figures are chosenfor convenience and clarity of presentation and are not necessarilyshown to scale.

FIG. 1 schematically shows a man operating a radar phone in a DF mode totransmit information to woman in his field of view whom he would like tomeet, in accordance with an embodiment of the present invention;

FIG. 2 schematically shows a block diagram of the radar phone DFcircuitry comprised in the radar phone operating in the DF mode shown inFIG. 1, in accordance with an embodiment of the present invention; and

FIG. 3 schematically shows a block diagram of RB circuitry comprised ina radar phone, such as that carried by the woman contacted by the manshown in FIG. 1, in accordance with an embodiment of the presentinvention;

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 schematically shows a man 20 operating a radar phone 22 totransmit information to a woman in his field of view whom he would liketo meet, in accordance with an embodiment of the present invention.

Radar phone 22 comprises two antennae 24 and 26 and direction finding(DF) circuitry coupled to the antennae that enable the radar phone tolocate the source of radio beacon (RB) signals transmitted by radarphones operating in a radio beacon mode. Features of the directionfinding circuitry and its integration with conventional cell phonecircuitry comprised in radar phone 22 are presented below in thediscussion of FIG. 2.

Operator 20 has selected from a menu (not shown) in his radar phone 22an option to operate the phone in a direction finding (DF) mode. In theDF mode, radar phone 22 periodically transmits an interrogation signal,schematically represented by dashed circles 25. Interrogation signal 25is transmitted over a DF channel, which may for example be a frequencyband that is a portion of the bandwidth used for conventional cellulartelephony or an ISM band. The signal is, optionally, substantiallynon-directional and may readily be received in a neighborhood ofoperator 20 that includes at least a portion of the operator's field ofview. Transmission of the interrogation signal is controlled by acontroller (shown in FIG. 2) in accordance with a DF transmissionalgorithm with which the controller is programmed.

By way of example, three people 31, 32 and 33 carrying radar phones 41,42 and 43 respectively are located in the field of view of operator 20and are operating in a radar beacon (RB) mode. Each phone 41, 42 and 43is capable of operating in the RB mode but, optionally, not in thedirection finding mode. As a result, each phone 41, 42 and 43 has asingle antenna 50 rather than two antennae, which are required for aradar phone in accordance with the invention to enable the radar phoneto operate in the DF mode. Features of RB circuitry comprised in radarphones 41, 42 and 43 and integration of the circuitry with conventionalcell phone circuitry comprised in the phones are presented below in thediscussion of FIG. 3.

In the RB mode each radar phone 41, 42 and 43 listens to its radioenvironment to detect presence of an interrogation signal. Upondetection of interrogation signal 25 transmitted by radar phone 22, eachradar phone 41, 42 and 43 attempts to respond to the interrogationsignal with a transmission of an RB signal comprising a CW portion andID data that identifies the phone. Transmission of the RB signal by aradar phone 41, 42 or 43 is controlled by a controller (FIG. 3)comprised in the cell phone in accordance with an RB transmissionalgorithm with which the controller is programmed. The RB transmissioncontrol algorithm for each radar phone 41, 42 and 43 optionally controlsthe radar phone to transmit its RB signal a predetermined number oftimes following an optionally fixed, predetermined delay period thatbegins, optionally, upon cessation of reception of interrogation signal25. Optionally, each radar phone 41, 42 and 43 transmits its RB signal asame number of times following reception of interrogation signal 25.Optionally, each radar phone transmits its RB signal once followingreception of interrogation signal 25.

In accordance with an embodiment of the present invention, the RBtransmission control algorithm that controls RB signal transmissions bya radar phone is programmed with a delay period selected from apreferably large number of different delay periods. Delay periods for RBtransmission control algorithms are selected to reduce a probabilitythat two radar phones operating in an RB mode that receive a sameinterrogation signal are programmed with a same delay time. Optionally,the RB transmission algorithm for a radar phone operating in an RB modedithers its programmed time delay. Dithering reduces a probability thattwo RB operating phones receiving a same interrogation signal andhaving, by chance, a same delay period will clash in attempting torespond to the interrogation signal.

In addition, the RB transmission control algorithm for each radar phone41, 42 and 43 controls the radar phone to listen to its radioenvironment to detect RB signals transmitted by other radar phonesoperating in an RB mode. If, during its delay period a radar phone 41,42 or 43 detects an RB signal from another radar phone, the algorithmcontrols the phone to wait before transmitting its RB signal for a timeequal to its delay period from a time at which the detected RB signalends.

As a result, in response to interrogation signal 25, radar phones-41, 42and 43 transmit their respective RB signals, represented by concentricdashed circles 44, in sequence, one after the other in order ofincreasing length of their respective delay periods. Each RB signal 44is substantially non-directional and comprises a CW transmission and,optionally in a header, data that identifies the phone 41, 42 or 43transmitting the RB signal.

The DF transmission algorithm that controls transmission ofinterrogation signals 44 by radar phone 22 controls the phone so thatonce the phone transmits an interrogation signal 25 it waits an“interrogation delay period” following reception of an RB signal 44before transmitting another interrogation signal. The interrogationdelay period is chosen sufficiently long so that radar phone 22 does nottransmit a second interrogation signal 25 following a firstinterrogation signal 25 until generally all phones 41, 42 and 43transmit their respective RB signals 44 responsive to the firstinterrogation signal. Optionally, the interrogation delay period is apredetermined constant time period.

A portion, represented by a block arrow 46, of the energy from each RBsignal 44 transmitted by a radar phone 41, 42 and 43 is received byradar phone 22. The DF circuitry in radar phone 22 uses the CW part ofthe received RB signal 44 from each radar phone 41, 42 and 43 todetermine an azimuth and range for the radar phone transmitting the RBsignal. Azimuth angle is determined relative to a plane midway betweenantennae 24 and 25 perpendicular to a plane that contains the antennae.A dashed line 28 indicates a line of intersection of the midway planeand the plane that contains the antennae. The DF circuitry transmits thedetermined azimuth and range for each radar phone 41, 42 and 43 to thecontroller comprised in radar phone 22.

The controller in radar phone 22 uses the determined azimuth and rangefor each radar phone 41, 42 and 43 to display an icon, i.e. an RB icon,representing the phone on a display screen 48 of DF radar phone 22. Aposition of the RB icon on the screen corresponds to the location of theradar phone 41, 42 and 43 that it represents in the field of view ofoperator 20. Optionally, the icon is displayed against a backgroundimage of a radar screen as schematically shown in FIG. 1. Icons 51, 52and 53 on display screen 48 represent respectively RB radar phones 41,42 and 43.

Of the three people 31, 32 and 33 in the field of view of operator 20,person 32 is a good-looking woman, whom the operator is interested inmeeting but is too shy to approach directly. Operator 20 compares thepositions of icons 51, 52 and 53 on display screen 48 with positions ofpeople 31, 32 and 33 in his field of view and determines that icon 52corresponds to woman 32. He composes an appropriate SMS messageincluding data that optionally includes his cell phone number andpersonal information that he hopes will convince woman 32 to use thecell phone number to call him. Operator 20 then selects icon 52 usingany of various methods known in the art, such as by repeatedly pressinga button on radar phone 22 to highlight each icon on display screen 48in turn until icon 52 is highlighted.

Upon selection of icon 52, operator 20 controls the DF circuitrycomprised in DF mode radar phone 22 to transmit the SMS message,optionally over a same DF communication channel that is used to transmitinterrogation signals 24 and RB signals 44. Optionally, the DF circuitrytransmits the SMS message over a radio communication channel differentfrom the DF channel. The different communication channel may be adedicated channel for radio transmission of data between DF mode radarphone 22 and RB mode radar phones 41, 42, and 43.

The SMS message is transmitted together with the ID data that identifiesradar phone 42 carried by woman 32, which ID data radar phone 22received with RB signal 44 transmitted by the woman's radar phone 42.The transmitted SMS message is received by all RB mode phones 41, 42 and43 in the field of view of operator 20. However, because the SMS messageis coded with the ID of RB radar phone 22, only RB radar phone 42 storesthe SMS message in a memory comprised in the phone. At her leisure, thewoman will read messages that her phone has stored and, hopefully foroperator 20, will be convinced by his message to contact him.

Whereas in the above scenario operator 20 optionally transmits his SMSmessage to phone 42 over the DF channel, or another radio channel, insome situations the operator 20 may transmit his message via aconventional cell telephony channel. For example, the ID data comprisedin RB signal 44 transmitted by RB phone 42 may comprise, instead of anID identifier of the RB phone, the cell phone number of the RB phone. Inthat case, operator 20 can use his phone 22 to transmit his message viaconventional cell telephony.

It is noted that in the above example there are only three people in thefield of view of operator 20 and he is readily able to associate RB icon52 with woman 32 from the few RB icons displayed on screen 48 of hisradar phone 22. In many situations however a person's field of view maybe crowded and it can be more difficult to associate a given person witha given icon.

In some situations an operator of DF mode phone may be aided inassociating a particular icon with a given person he or she isinterested in contacting by movement of the given person in theoperators field of view. For example, if woman 32 was moving in aparticular direction, operator 20 would be aided by corresponding motionof icon 52 on the screen of his radar phone that icon 52 corresponds tothe woman.

In some embodiments of the invention ID data comprised in RB signals 44transmitted by radar phones operating in an RB mode comprise visual cuedata that aid in determining which icons displayed on screen 48 belongto which people in the field of view of operator 20. For example, assumethat woman 32 is a red head. She may program her radar phone 42 so thatRB signals 44 transmitted by the phone comprises ID data indicating thecolor of her hair. Operator 20 may access, inter alia, visual cue datafor each RB icon on his screen using suitable options provided by thecontroller in his radar phone 22. Assuming that other people in theimmediate vicinity of woman 32 are not red heads, operator 20 canreadily associate with woman 32 that icon on his screen whose IDcomprises “red hair”. Visual cue data can be advantageous in crowdedsituations for which many people carrying RB node phones responding tointerrogation signals are located close to each other.

In some embodiments of the present invention, interrogation signalstransmitted by a DF phone may comprise ID data associated with theoperator of the DF phone. An RB phone may be programmed by its owner totransmit an RB signal only if the ID data in the interrogation signalcorresponds to the RB phone's personal preferences. For example, the IDdata in an interrogation signal may include data indicating that theowner of the DF phone transmitting the interrogation signal is a smoker.The RB phone may be programmed to respond with an RB signal only tonon-smokers.

Interrogation signals may also comprise “preference data” responsive towhich RB phones determine whether to transmit or not to transmit an RBsignal, in accordance with an embodiment of the present invention. Forexample, an interrogation signal may indicate that an RB radar phonerespond to the interrogation signal only if the RB phones owner in anon-smoker.

Programming DF and RB radar phones with their users' preferences inaccordance with an embodiment of the invention improves efficiency of“match-making” provided by the radar phones. In addition the preferencesin general reduce a number of RB phones that respond to interrogationsignals transmitted by a given DF phone in a given situation. As aresult, the given DF phone's screen will in general be less cluttered,and a user of the DF phone will find it easier to associate of RB iconsdisplayed on the screen with people in the user's field of view.

In some situations, more than one person may be interested in operatinga radar phone in a DF mode in a same region. RB signals transmitted by aradar phone operating in an RB mode are substantially non-directional.As a result, generally all radar phones of a plurality of radar phonesoperating in a DF mode in a same region will receive RB signalsgenerated in response to an interrogation signal transmitted by any oneof the DF mode radar phones. All the DF mode radar phones operating inthe region will therefore be able to locate RB mode phones in the regionthat transmit RB signals responsive to the interrogation signalstransmitted by the one DF mode radar phone. In general therefore, if aplurality of DF mode phones are operating in a same region, it issufficient for a single one of the plurality of DF mode phones totransmit interrogation signals.

In accordance with an embodiment of the present invention, the DFalgorithm in accordance with which DF operation of a DF mode radar phoneis controlled suppresses transmission of interrogation signals by theradar phone if the phone senses interrogation signals are beingtransmitted by another DF mode phone. In some embodiments of the presentinvention a radar phone operating in a DF mode initiates transmission ofinterrogation signals if it does not sense an interrogation signalduring a predetermined “waiting” period.

In some situations, the field of view of a first person operating afirst radar phone in a DF mode may overlap but not be identical to thefield of view of a second person operating a second DF phone in a DFmode. Transmission of interrogation signals by the first radar phone mayprevent the second radar phone from transmitting interrogation signalsresulting in the field of view of the second person not beingsatisfactorily covered by interrogation signal transmissions.

In accordance with an embodiment of the invention, the second radarphone generates a signal to alert the second person that RB signals thatthe second phone receives are not generated responsive to interrogationsignals transmitted by the second phone. Responsive to the signal thesecond person may control his or her phone to transmit an interruptsignal requesting that the first phone cease transmission ofinterrogation signals. In some embodiments of the invention, uponreceipt of the interrupt request signal the first phone generates asignal to alert the first person to the interrupt request, responsive towhich the first person may defer to the request. In deferring to therequest, the first person may control the first phone to stop operatingin the DF mode or operate in a DF mode without transmittinginterrogation signals.

In some embodiments of the invention a radar phone operating in a DFmode that receives an interrupt request automatically defers to therequest and following an appropriate “interrupt delay” generates its owninterrupt transmission requesting return of permission to transmitinterrogation signals. In accordance with an embodiment of theinvention, interrupt delays and interrogation delay periods of aplurality of radar phones operating in a same region in a DF mode arecontrolled so that the radar phones smoothly hand off one to the otherthe task of transmitting interrogation signals.

FIG. 2 schematically shows a block diagram, in accordance with anembodiment of the present invention, of component circuitry comprised inradar cell phone 22 operated by man 20 in FIG. 1. Radar phone 22comprises a cell phone front end 60 for transmitting and receivingconventional cell phone telephone signals, a direction finding (DF)module 62 and a controller 64 connected to the front end and to the DFmodule. DF module 62 comprises a transceiver (not shown) fortransmitting interrogation signals and receiving RB signals anddirection finding circuitry for determining an azimuth and optionally arange for a radar phone transmitting an RB signal.

Cell phone front end 60 is permanently connected to antenna 26 and DFmodule 62 is permanently connected to antenna 24. DF module 62 is alsoconnected to a switch 66, optionally via a narrow band filter 68. Switch66 is controllable by controller 64 to selectably connect and disconnectDF module 62 (via filter 66) to antenna 66. Narrow band filter 68substantially blocks cell telephony signals and transmits signals in theDF communication channel. Filter 68 substantially prevents DF module 62from receiving energy from cell telephony signals received at antenna 26and reduces loading of antenna 26 at cell telephony signal frequencieswhen switch 66 connects DF module 62 to the antenna. Controller 64 andfront end 60 comprise conventional telephony circuitry and operate toprovide conventional cell telephony communications. In the DF mode,controller 64 controls switch 66 to connect DF module 62 to antenna 26and controls the DF module to transmit interrogation signals 25 andreceive radar beacon signals 44 as described above.

Optionally, the direction finding circuitry in DF module 62 isWatson-Watt direction finding circuitry. Prior art Watson-Watt directionfinders conventionally require antennae coupled to a relatively largeground plane having dimensions that would appear to prohibitincorporating a Watson-Watt direction finder in a cell-phone shellhaving acceptable and convenient dimensions. Watson-Watt directionfinders and the theory of operation of conventional Watson-Wattdirection finders are discussed in an article entitled “Basics of theWatson-Watt Radio Direction Finding Technique”, Web Note WN-002,provided by RDF Products of the U.S.; [online]; [retrieved on May 23,2003]; retrieved from the internet <URL:www.rdfproducts.com>, thedisclosure of which is incorporated herein by reference. In spite ofprior art convention, the inventors have found that it is possible toprovide a Watson-Watt direction finder having antennae coupled to arelatively small ground plane, which has dimensions suitable forpackaging in a conventional cell phone shell. In FIG. 2, antennae 24 and26 of cell phone 22 are coupled to a relatively small ground planeschematically indicated by dashed line 70.

To provide a relatively small Watson-Watt direction finder, inaccordance with an embodiment of the present invention, preferablydirection-finding signals are transmitted at relatively highfrequencies. For example, DF signals transmitted with carrierfrequencies close to carrier frequencies in the ISM (800 MHz-900 MHz)bands are suitable for operating a Watson-Watt direction finder smallenough to be conveniently incorporated in a cell-phone shell. Theinventors have found that such a small “high frequency” Watson-Wattdirection finder can be configured to provide satisfactory azimuth andrange data for RB phones operating in a range from about 5 meters toabout 100 meters from a DF phone comprising the Watson-Watt directionfinder.

The above noted maximum range limitation of the Watson-Watt directionfinder is determined substantially by a maximum power allowed fortransmission of signals over the ISM band. Ranges in excess of 100meters for reception of RB and interrogation signals are possibleprovided the signals are transmitted at appropriate power levels. Fortypical use such as indoors in a room or lecture hall, or outdoor usefor example at a lawn party, a maximum operating range of 50 or 100meters is generally satisfactory.

In accordance with an embodiment of the invention, the Watson-Wattdirection finder operates with two antennae, antennae 24 and 26.Optionally, a difference in signal attenuation over leads that connectantennae 24 and 26 to DF module 62 is less than about 0.3 dB.Optionally, the difference in signal attenuation is less than about 0.2dB. Optionally, the difference in signal attenuation is less than about0.1 dB.

DF module 62 determines azimuth for an RB phone 41, 42 or 43 responsiveto a difference between the phase and/or amplitude of RB signal 44received from the RB phone at antennae 24 and 26. Optionally, DF module62 determines a DC level of the received RB signal and determines arange for the RB phone responsive to the DC level. The range and azimuthdetermined for an RB phone are transmitted to controller 64, which usesthe azimuth and optionally the range to display an icon representing theradar phone transmitting the RB signal on display screen 48 of radarphone 22.

Typical Watson-Watt direction finders coupled to two antennae compriseone quarter wavelength antennae spaced apart by about a half carrierwave wavelength. Direction finding is achieved by rotating the antennaeto locate a direction in which a difference in a signal phase oramplitude at the two antennae are a minimum. A direction, i.e. azimuth,at which the difference is a minimum, is a direction along which thesource of the signal is located. Such Watson-Watt direction finders havea relatively narrow azimuth angle dynamic range and can functionsatisfactorily with the narrow dynamic range because the antennae arerotated. In general, they have an effective dynamic range of about 30°and are sensitive to azimuth changes in the location of a signal sourcein a range of angles between about 15° on either side of a plane midwaybetween the antennae and perpendicular to a plane that contains theantennae.

A field of view of a person generally extends in azimuth from about 90°to the right of a person to about 90° to the left of the person. For adirection finder, in accordance with the invention that displayslocations of RB phones in a field of view of a person, it isadvantageous for the direction finder to have an effective azimuth angledynamic range larger than the typical dynamic range of prior artWatson-Watt direction finders.

The inventors have found that an antennae configuration for aWatson-Watt direction finder in which the antennae are shorter and moreclosely spaced than in a conventional prior art Watson-Watt directionfinder provides increased azimuth angle dynamic range for the directionfinder. In particular, for the ISM frequencies (800 MHz-900 MHz),electrical lengths of antenna 24 and 26 are advantageously less thanabout one fifth of the wavelength of an RB signal carrier wave and arespaced apart by less than about one fifth of the carrier wavelength.Preferably, the electrical lengths of antennae 24 and 26 are equal toabout one sixth the wavelength of the carrier wave and are spaced apartby a distance equal to about one eighth the carrier wavelength. For theISM frequencies and the relatively short and closely spaced antennae 24and 26, a difference in amplitude of an RB signal received at antennae24 and 26 from an RB phone is substantially linear with azimuth of theRB phone's location for a range of azimuths greater than the azimuthdynamic range of most conventional Watson-Watts configurations. Theinventors have found that for a person holding a DF cell phonerelatively perpendicular to the ground and between about 25 and 40centimeters from his or her body, the azimuth may be substantiallylinear with amplitude difference for azimuths between about −60° toabout +60°. In some instances the linear range may extend from about−90° to about +90°. (The slope of the substantially linear relationshipbetween azimuth and amplitude difference for azimuths from 0° to about−60° or −90° is opposite to the slope of the relationship between 0° andabout +60° or +90°.)

In addition to the advantages of increased azimuth angle dynamic range,the inventors have also found that for the relatively short and closelyspaced antennae 24 and 26, sensitivity of azimuths and ranges determinedby DF module 62 to orientation of DF cell phone 22 is reduced. That is,direction finding is relatively less sensitive to how accurately cellphone 22 in FIG. 1 is held with antennae 24 and 26 perpendicular to theground.

It is noted that a Watson-Watt direction finder coupled to two antennaedoes not distinguished between the location of signal sources that aremirror images of each other in a plane in which the antennae arelocated. For example, using the hours of a clock to indicate direction,assume that the plane containing antennae 24 and 26 are perpendicular tothe face of the clock and intersects 3 o'clock and 9 o'clock. Assumefurther that the face of the clock is parallel to the ground so thatantennae 24 and 26 and axis 28 (FIG. 1) are perpendicular to the ground.An RB phone at 12 o'clock and an RB phone at 6 o'clock would have RBicons at a same position on screen 48 (FIG. 1). Similarly, RB phones at2 o'clock and at 4 o'clock would have icons at a same location on screen48. Mirror image RB phones may be distinguished by operator 20 rotatinghis phone about axis 28 (FIG. 1). The RB phones at 12 and 2 o'clock willmove right and the RB phones at 4 and 6 o'clock will move left.

Since cell phone front end 60 is permanently connected to antenna 26radar phone 22 is always able to receive incoming telephony signals andby default, optionally, controller 64 controls circuitry in the radarphone so telephony functions override all DF functions of the radarphone. For example, if a telephony signal is received by radar phone 22the radar phone interrupts DF mode functioning of the phone that may bein progress and controls switch 66 to disconnect DF module 62 fromantenna 26. It is noted, that since filter 68 substantially blockstelephony signals DF module 62 substantially does not interfere withincoming or outgoing telephony signals received or transmittedrespectively via antenna 26.

FIG. 3 schematically shows a block diagram of component circuitrycomprised in radar phone 41 as well as in radar phones 42 and 43 (FIG.1). Radar phone 41 comprises a cell phone front end 60 for transmittingand receiving conventional cell phone telephone signals, an RFtransceiver 72 and a controller 64 connected to the front end and to thetransceiver.

Since radar phone 41, optionally operates only in an RB mode (and ofcourse a conventional telephony mode), as noted above the radar phonedoes not require two antennae and has only a single antenna 26. Singleantenna 26 supports both conventional cell telephony and radio beaconmodes. Similarly, to circuitry in radar phone 22, radar phone 41comprises a cell phone front end 60 and controller 64 that supportconventional cell phone telephony. Cell phone front end is permanentlyconnected to antenna 26. Optionally, a narrow band filter 68) protectsRF transceiver 72 and substantially prevents the transceiver fromreceiving energy from energy comprised in telephony signals.

Similarly to DF module 62 in cell phone 22 transceiver 72 is connectedto a switch 66 optionally via a narrow band filter 68. Switch 66 iscontrolled by controller 64 to selectably connect and disconnecttransceiver 72 from antenna 26. In the RB mode, controller 64 controlsswitch 66 to connect transceiver 72 to antenna 26 and controls thetransceiver to transmit radar beacon signals 44 over the DF channel inresponse to received interrogation signals 25 as described above.

In the above description radar phone 22 is described as operating in aDF mode and radar phones 41, 42 and 43 are described as operating in anRB mode but not having capability of operating in a DF mode. In someembodiments of the invention a radar phone may selectively be operablein either a DF mode or an RB mode. For example, a radar phone such asradar phone 22 having a DF module and capable of operating in a DF modemay also be operable in an RB mode. In the RB mode controller 64disconnects DF module 62 from antenna 26 and receives interrogationsignals via antenna 24 and the RF transceiver comprised in the DFmodule. Controller 64 controls the RF transceiver to transmit RB signalsin response to received interrogation signals in accordance with an RBtransmission algorithm as described above.

It is noted that direction finding module 62 and radio beacon circuitry72, i.e. RF transceiver 72, are comprised in cell phones havingcircuitry used for conventional cell phone telephony. The presentinvention is not limited to direction finding circuitry incorporated incell phones and direction finding circuitry in accordance with anembodiment of the present invention may be used independently of cellphones and cell phone circuitry.

A hand-holdable direction finding communication unit, in accordance withan embodiment of the present invention, may for example comprisedirection finding components and features similar to those found in cellphone 22 but not circuitry that provides conventional cell phonetelephony functions (e.g. cell phone front end 60). A radio beaconcommunication unit, in accordance with an embodiment of the presentinvention, may similarly comprise components and features comprised inRB phone 41 but not circuitry for cell telephony functions. Similarly tocommunication between radar phones described above, transmission of dataand SMS messages between direction finding units, hereinafter “DFunits”, and radio beacon units, hereinafter “RB units”, may be providedover a same a DF communication channel used for direction finding orover a dedicated radio transmission channel.

By way of example of a use of DF and RB units that do not comprisetelephony functions, such units may be handed out at a convention orceremony to facilitate communication and contact between peopleattending the convention or ceremony. Since, as in the case of DFphones, which can also optionally function as RB phones, DF units canoptionally also function as RB units and it may be expected that at aceremony or convention only DF units that also function as RB units maybe handed out.

By way of another example, DF and RB units may also be used forfunctions that do not involve only people. For example, a bird or moosehunter who hunts with dogs generally wants to known where his dogs arebut may not be able to maintain visual contact with the dogs. Systemsfor tracking a hunter's dogs are known in the art. Pointer PositionSolutions of Finland markets a system that enables a hunter who operatesa handheld direction finder to determine an azimuth of a dog wearing atransmitter that transmits a radio beacon. The direction findercomprises circuitry for receiving the radio beacon and indicatingstrength of the received radio beacon signal. The hunter rotates thedirection finder until the signal strength that it indicates is amaximum. A direction in which the direction finder points when themaximum is indicated is an azimuth of the dog. It is noted that thedirection finder determines magnitude of a radio beacon signaltransmitted from the dog's transmitter but not an azimuth for the dog.The hunter determines the azimuth from the direction in which the radiobeacon signal is a maximum. The direction finder may be used to track upto two dogs but “switches” between the dogs and tracks only one dog at atime.

The company also markets a direction finding system for locating dogsusing GPS. A hunter carries a GSM phone comprising a GPS-map navigator.A dog being tracked by the system wears a GSM/GPS transceiver thatdetermines the dog's position using GPS satellite signal and transmitsthe position via a GSM cell telephony network to the hunter's phone. Thedog's position may be displayed against a background map of a region inwhich the hunter and dog are hunting. Because a GPS receiver requires adirect line of sight between the receiver and at least three GPSsatellites in order to determine its position, the system will not beable to update the dog's location for locations for which the threelines of sight do not exist. For example, if a dog being tracked wandersinto a barn or thick underbrush the system will generally not be able toupdate the dog's position. For the same reason the Pointer dog-GPSsystem not is advantageously used for indoor tracking of pets. Pointerproducts are described in the company's product data sheets [online];[retrieved on May 20, 2003]; retrieved from the internet<URL:www.pointersolutions.com>.

A dog tracking system that comprises DF and RB units in accordance withthe present invention on the other hand may be used to provide a hunterwith an azimuth and range for each of a plurality of greater than twodogs and can also operate indoors. For example, an RB unit may bemounted to each of the hunter's dogs and the hunter may operate a DFunit to determine an azimuth and range for each of the dogs from theradio beacon transmitted by the dog's RB unit. The azimuths and rangesfor the dogs are used to display the positions of the dogs on the DFunits “radar screen”. For locating dogs in a typical hunting environmentit may be advantageous to transmit RB signals at power levels thatenable effective detection of the signals at ranges in excess of 100meters. For example a range of about 200 meters may be advantageous. Toaid in identifying each dog, the DF unit may generate a different RBicon on the screen of the DF unit for each dog responsive to ID datacomprised in RB signals transmitted by the RB unit attached to the dog.

In some embodiments of a “direction finding” communication system inaccordance with the invention, cell phones used in the system compriseGPS receivers to identify and determine locations of persons. Inaddition, the cell phones comprise a communication transceiver fortransmitting non-telephony signals and optionally a magnetic compassand/or a GPS compass. Optionally the transceiver is an RF transceiver.Optionally the transceiver is a blue tooth transceiver. The GPS receiverin a cell phone determines and periodically updates its position, andthereby that of its operator, responsive to signals received from GPSsatellites. The compass in a cell phone is used for determining aheading of the phone's operator. Each phone may selectably be operatedin a “direction query” (DQ) mode or in a “query response” (QR) mode.

If a first person, such as for example operator 20 in FIG. 1, wants toaccess a second person, such as woman 32, the first person controls hisphone to operate in the DQ mode. In the DQ mode, the transceiver in thefirst person's phone transmits non-directional interrogation signals,which are received by phones carried by people operating in a QR modeand that are within range of the transceiver.

Interrogation signals transmitted by the DQ phone are similar tointerrogation signals transmitted by a DF phone, (e.g. DF phone 22)described above and may comprise any of the data and/or configurationsof data, such as for example ID and/or preference data, comprised byinterrogation signals transmitted by the DF phone. A QR phone thatreceives the interrogation signals may respond by controlling itstransceiver to transmit non-directional QR signals. A decision of the QRphone whether or not to respond to the interrogation signals isoptionally determined similarly to a manner in which a DF phonedetermines whether or not to respond to an interrogation signal. Forexample the QR phone optionally determines whether to respond to theinterrogation signals responsive to data comprised in the interrogationsignals and preferences with which the QR phone is programmed. Timing oftransmission of interrogation signals transmitted by DQ phones andtransmission of QR signals in response to the interrogation signals byQR phones is optionally controlled similarly to the way timing oftransmission of DF and RB signals is controlled.

QR signals transmitted by the QR phone optionally comprise data and/orconfigurations of data, such as ID data and preference data that iscomprised in RB signals transmitted by RB phones described above. Inaddition however, the QR signals comprise spatial coordinates determinedby the QR phone's GPS receiver. The DQ phone uses the GPS coordinatescomprised in QR signals that it receives to display on the DQ phone'sdisplay screen, “position icons” representing the QR phones thattransmit the QR signals. An icon representing a QR phone has a locationon the DQ phone's display screen that is homologous with the actualspatial location of the QR phone.

To aid the first person operating the DQ phone in determining positionsof the responding QR phones relative to his or her own position, the DQphone displays, together with the position icons, the first person'sheading. The heading is determined responsive to data generated by theDQ phone's magnetic and/or GPS compass. Optionally, the position iconsare displayed against a background of a radar screen. Optionally, theposition icons are displayed against a background of a map of the firstperson's surroundings.

The first person uses the display of position icons on his DQ phone'sdisplay screen to identify a person, e.g. woman 32, whom he would liketo contact, similarly to the way in which operator 20 uses the displayon DF phone 22 (FIG. 1). Transmission of information to the person ofhis choice is accomplished by RF transmission using the DF phone'stransceiver and/or by telephony transmission, as described above.

In the description and claims of the present application, each of theverbs, “comprise” “include” and “have”, and conjugates thereof, are usedto indicate that the object or objects of the verb are not necessarily acomplete listing of members, components, elements or parts of thesubject or subjects of the verb.

The present invention has been described using detailed descriptions ofembodiments thereof that are provided by way of example and are notintended to limit the scope of the invention The described embodimentscomprise different features, not all of which are required in allembodiments of the invention. Some embodiments of the present inventionutilize only some of the features or possible combinations of thefeatures. Variations of embodiments of the present invention that aredescribed and embodiments of the present invention comprising differentcombinations of features noted in the described embodiments will occurto persons of the art. The scope of the invention is limited only by thefollowing claims.

1. A direction finding system comprising: at least one first handholdable unit comprising circuitry that transmits a radio beacon signal;and at least one second hand holdable unit having a display screen andcomprising direction finding (DF) circuitry that receives a radio beacon(RB) signal transmitted by a first unit of the at least one first unitand determines from the received radio beacon signal an azimuth anglefor the location of the first unit; wherein the controller generates adisplay on the display screen responsive to the azimuth angle thatindicates a location of the first unit.
 2. A direction finding systemaccording to claim 2 wherein the direction finding circuitry comprisesWatson-Watts direction finding circuitry.
 3. A direction finding systemaccording to claim 2 wherein for receiving RB signals the at least onesecond unit comprises a first antennae and a second antenna electricallyconnected to the Watson-Watts direction circuitry.
 4. A directionfinding system according to claim 3 wherein a difference in signalattenuation between the electrical connections of the antennae to theWatson-Watts circuitry is less than about 0.3 dB.
 5. A direction findingsystem according to claim 3 wherein a difference in signal attenuationbetween the electrical connections of the antennae to the Watson-Wattscircuitry is less than about 0.2 dB.
 6. A direction finding systemaccording to claim 3 wherein a difference in signal attenuation betweenthe electrical connections of the antennae to the Watson-Watts circuitryis less than about 0.1 dB.
 7. A direction finding system according toclaim 3 wherein the antennae have an electrical length less than aboutone fifth the wavelength of a carrier wave of the radio beacon signal.8. A direction finding system according to claim 7 wherein the antennaehave an electrical length equal to about one sixth the wavelength of thecarrier wave of the radio beacon signal.
 9. A direction finding systemaccording to claim 3 wherein the two antennae are spaced apart by adistance less than about one fifth of the carrier wavelength.
 10. Adirection finding system according to claim 3 wherein the two antennaeare spaced apart by a distance equal to about one eighth of the carrierwavelength.
 11. A direction finding system according to claim 3 whereinthe Watson-Watts circuitry determines the azimuth from a differencebetween amplitude and/or phase of the received RB signal at theantennae.
 12. A direction finding system according to claim 2 whereinthe at least one first unit comprises circuitry and apparatus thatprovides conventional cell phone telephony.
 13. A direction findingsystem according to claim 12 wherein the at least one first unitcomprises a common antenna for transmitting RB signals and for cellphone telephony functions.
 14. A direction finding system according toclaim 13 wherein the at least one first unit comprises a switchcontrollable to selectably, electrically connect the common antenna tothe radio beacon circuitry or the cell phone circuitry.
 15. A directionfinding system according to claim 3 wherein the at least one second unitcomprises circuitry and apparatus that provides conventional cell phonetelephony.
 16. A direction finding system according to claim 15 whereinthe at least one second unit comprises a switch controllable toselectably, electrically connect the first antenna to the directionfinding circuitry or the cell phone circuitry.
 17. A direction findingsystem according to claim 12 wherein the RB signals comprise a carrierwave and the at least one first unit and the at least one second unitcomprise a filter that blocks electromagnetic energy at a frequency ofthe carrier wave from reaching the cell phone circuitry.
 18. A directionfinding system according to claim 1 wherein the at least one first unitcomprises circuitry and apparatus that provides conventional cell phonetelephony.
 19. A direction finding system according to claim 1 whereinthe at least one second unit comprises circuitry and apparatus thatprovides conventional cell phone telephony.
 20. A direction findingsystem according to claim 1 wherein the direction finding circuitrydetermines a range for the first unit of the at least one unitresponsive to the received RB signal.
 21. A direction finding systemaccording to claim 20 wherein the direction finding circuitry determinesa DC level of the RB signal.
 22. A direction finding system according toclaim 21 wherein the controller determines the range responsive tomagnitude of the DC level.
 23. A direction finding system according toclaim 20 wherein the controller generates the display responsive to thedetermined range.
 24. A direction finding system according to claim 1wherein the at least one second unit comprises circuitry fortransmitting signals to the at least one first unit and the at least onefirst unit comprises circuitry for receiving signals transmitted by theat least one second unit.
 25. A direction finding system according toclaim 24 wherein a second unit of the at least one second unit transmitsan interrogation signal responsive to which a first unit of the at leastone first unit that receives the interrogation signal transmits an RBsignal.
 26. A direction finding system according to claim 25 whereinsubsequent to transmitting the interrogation signal the second unittransmits at least one additional interrogation signal.
 27. A directionfinding system according to claim 26 wherein each of the at least oneadditional interrogation signal is transmitted following a delay periodthat begins after a last RB signal received by the second unit that istransmitted by the at least one first unit responsive to the precedinginterrogation signal.
 28. A direction finding system according to claim25 wherein each interrogation signal transmitted by the second unitcomprises ID data specific to a user of the second unit.
 29. A directionfinding system according to claim 28 wherein each of the at least onefirst unit is programmable with preference data specific to a user ofthe first unit and if it receives an interrogation signal transmitted bythe second unit it transmits an RB signal responsive thereto only if theID data in the transmitted interrogation signal matches preference datawith which it is programmed.
 30. A direction finding system according toclaim 25 wherein the transmitting circuitry of each first unit transmitsits RB signal following a predetermined delay period after receipt of aninterrogation signal.
 31. A direction finding system according to claim30 wherein the predetermined delay period for each first unit is chosenfrom plurality of different delay periods so as to reduce a probabilitythat any two of the first units that receive a same interrogation signalhave a same delay period.
 32. A direction finding system according toclaim 30 wherein the transmitting circuitry of the first unit dithersits predetermined delay period.
 33. A direction finding system accordingto claim 1 wherein each first unit is programmable so that RB signalstransmitted by the first unit comprises ID data specific to a user ofthe first unit.
 34. A direction finding system according to claim 33wherein each unit of the at least one second unit is controllable by itsuser to transmit a signal comprising ID data that it receives in an RBsignal from a given first unit whose location is indicated in thedisplay, which given first unit is selectable by the user from thedisplay.
 35. A direction finding system according to claim 34 whereinthe second unit is programmable with preference data specific to thesecond unit's user and wherein the location of a first unit is indicatedon the screen only if ID data in the RB signal received from the firstunit matches preference data with which it is programmed.
 36. Adirection finding system according to claim 1 wherein the displayindicating a position of a first unit comprises an icon representing thefirst unit displayed against a background of a radar screen and whereina location of the icon on the radar screen corresponds to a location ofthe first unit relative to the orientation of the second unit.
 37. Adirection finding system according to claim 36 wherein a first unit ofthe at least one first unit is programmable so that RB signals that ittransmits comprises data encoding at least one visual cue characteristicof the user of the first unit.
 38. A direction finding system accordingto claim 37 wherein the controller of the at least one second unitdisplays on the screen, in association with an icon representing a firstunit, a visual cue of the at least one visual cue encoded in an RBsignal it receives from the first unit.
 39. A direction finding systemaccording to claim 1 wherein the RB signals comprise a carrier wavehaving a frequency in a range from about 800 MHz to about 900 MHz.
 40. Adirection finding system according to claim 1 wherein a second unit ofthe at least one second unit has an effective maximum range less than orequal to about 200 meters for receiving an RB signal transmitted by afirst unit that can be used to determine an azimuth for the first unit.41. A direction finding system according to claim 40 wherein the maximumrange is less than or equal to about 100 meters.
 42. A direction findingsystem according to claim 41 wherein the maximum range is less than orequal to about 50 meters.
 43. A communication system comprising: aplurality of cellular phones each of which comprises a display screen aGPS receiver that determines spatial coordinates for the phone'sposition and a transceiver for transmitting non-telephony signals;wherein the transceiver of a first phone of the plurality of phones iscontrollable to transmit an interrogation signal responsive to which thetransceiver of a second phone of the plurality of phones that receivesthe interrogation signal transmits a signal comprising GPS coordinatesof the second phone; and if the first phone receives the signaltransmitted by the second phone, it displays a position icon responsiveto the GPS coordinates on the first phone's screen that indicates alocation of the second phone.
 44. A communication system according toclaim 43 wherein each phone comprises a compass that generates signalsresponsive to a heading of an operator of the phone and wherein thesecond phone displays responsive to the compass signals, and togetherwith the position icon, a heading icon indicating the heading of thesecond phone's operator.
 45. A communication system according to claim44 wherein the compass comprises a GPS compass.
 46. A communicationsystem according to claim 44 wherein the compass comprises a magneticcompass.
 47. A direction finding system according to claim 2 wherein theat least one second unit comprises circuitry and apparatus that providesconventional cell phone telephony.